How to choose the right capacitor for any application

Aluminum? Ceramic? Film? Mica? The best type of capacitor for your circuit isn’t always clear, but this list of 17 common capacitor applications will help you narrow it down.

Capacitors are one of the most basic circuit elements that electronic engineers can use. But basic doesn’t mean simple. There’s a rich variety of capacitor types and ways to use them, and even seasoned engineers may need some help in pairing the right capacitor with the right circuit.

The engineer’s complete guide to capacitors aims to provide that help. Throughout this series, we’ll examine the most popular types of capacitors and the most common capacitor applications, helping you choose the most effective capacitor no matter your requirements. This guide is meant for any engineer with capacitor questions, covering the basics as well as advanced use cases, so feel free to skip around to find the specific answers you’re looking for. A full overview of the guide is available here, and here’s a glossary of capacitor terms and acronyms.

This article provides a broad overview of the most common capacitor applications and the types of capacitors that best suit them. It covers:

  1. Buffer capacitors
  2. Bypassing capacitors
  3. Coupling capacitors
  4. Decoupling capacitors
  5. EMI/EMC capacitors
  6. Energy storage capacitors
  7. Feedback capacitors
  8. Filtering capacitors
  9. Frequency compensation capacitors
  10. Motor starting capacitors
  11. Power factor correction capacitors
  12. Power conditioning capacitors
  13. Pulsed power capacitors
  14. Resonant circuit capacitors
  15. Run capacitors
  16. Safety capacitors (EMI/RFI suppression capacitors and AC line filter capacitors)
  17. Snubber capacitors

You’ll find more information on the various types of capacitors in upcoming entries to this guide.

Buffer capacitors

A buffer capacitor is a capacitor placed in parallel with electrical contacts to provide arc suppression. Arcs are usually associated with inductive loads and can produce pitting that will limit contact life.

Buffer capacitors are physically large capacitors, rated to 1000 volts or more, and are used in big machines to stop contact arcing. They’re normally found in high-power transmission systems.

Bypassing capacitors

A bypassing capacitor often provides a low-impedance path to ground. It can be used to keep noise out of a load. It can also be used to shunt a signal around a gain-setting resistor to modify a circuit’s voltage gain as a function of frequency. In the latter case, a bypassing capacitor may not have one end tied to ground, but simply be in parallel with a gain-setting resistor. This permits the frequency response of an amplifier to be tailored to be less than that provided by its gain-bandwidth product.

Bypassing capacitor selection depends on your requirement specifications. Low-frequency applications can be served by aluminum electrolytics or tantalum electrolytics. Class 2 ceramic capacitors provide a volumetric efficiency advantage for non-critical applications like higher frequency bypassing.

Coupling capacitors

A coupling capacitor is used to provide a low-impedance path to connect a signal source to a signal-processing stage input, between processing stages (for example, the output of one stage to the input of the next stage) and from the ultimate output of the signal-processing system to the load.

In choosing coupling capacitors for audio frequency work, aluminum electrolytics or tantalum capacitors may be a good option. Niobium electrolytic capacitors may suit low-voltage applications (10 volts or less) with safety concerns. Higher voltage applications and operation at higher frequency may require Class 2 ceramic capacitors.

Decoupling capacitors

Decoupling capacitors are usually connected between the DC power supply (e.g., VCC) and ground. In the case of decoupling capacitors used with digital integrated circuits, the energy storage of the decoupling capacitor is used to hold the voltage across the digital integrated circuit constant. This ensures the digital integrated circuit operates properly and does not introduce noise in the DC power supply. Noise in the DC power supply can cause other circuits to operate erratically. Decoupling capacitors are also used in analog signal processing to prevent noise and reduce the possibility of interstage oscillation.

In audio frequency work, aluminum electrolytics or tantalum capacitors may be a good selection for decoupling capacitors. Low-voltage applications (10 volts or less) with safety concerns may be satisfied using niobium electrolytics. Higher voltage applications and operation at higher frequency, like digital ICs, may require Class 2 ceramic capacitors.

EMI/EMC capacitors

Capacitors are often used to prevent electromagnetic interference (EMI) and to implement electromagnetic control (EMC). Generally, these capacitors work well at high frequencies. The goals are to keep EMI noise from entering and disturbing a given system and to prevent that system from radiating EMI that could affect other systems.

Feedthrough capacitors are used commonly for EMI and EMC. Ceramic and film capacitors are the fundamental types used for Class X and Class Y safety capacitors. Ceramic capacitors provide higher capacitor values in a smaller volume, and are usually selected for low-power applications because of their smaller size.

Film capacitors exhibit self-healing, the ability of a metallized capacitor to clear a fault area where a momentary short occurs due to dielectric breakdown during an over-voltage condition. This ability is influenced by several factors, including the type of dielectric. Dielectrics such as polycarbonate and polypropylene have high surface oxygen content. This is an important factor, since oxygen is necessary to vaporize or “burn-off” the electrode around the fault area. PPS (polyphenylene film) is an example of a dielectric which does not have high surface oxygen content and therefore has a very low potential for self-healing. Higher capacitance safety capacitor values are only found in film technology.

Energy storage capacitors

All charged capacitors provide stored potential energy by virtue of the electric field directed from the positive plate to the negative plate. Stored energy levels can be small, such as those associated with analog sample-and-hold circuits and peak detector applications, or extremely large. Large energy storage applications can include strobe lights and automotive power audio systems. Extremely large energy storage requirements are associated with pulsed laser applications, rail guns and power grid energy storage.

Low-level energy storage in peak detector and sample-and-hold circuits should employ polystyrene capacitors because of their low dielectric absorption characteristic. Large energy storage requirements can be satisfied by aluminum electrolytic capacitors or supercapacitors.

Feedback capacitors

Capacitors are used to form negative feedback in op amp integrators. Feedback capacitors are also incorporated to limit the corner frequency of an op amp amplifier to a value below that determined by its gain-bandwidth product. In both cases the capacitors should have low leakage current and have adequate precision.

The best choices for feedback capacitors are class 1 ceramic capacitors, polystyrene film capacitors, and for high temperature applications, polycarbonate film capacitors.

Filtering capacitors

Low-pass, high-pass, band-pass and band-reject filters can be implemented with passive devices exclusively or with a combination of passive devices and active devices (for example, op amps). These realizations usually involve relatively low energy levels. In power supply applications, large-valued filtering capacitors are used for power conditioning to smooth out the pulsating DC produced by the rectifier stage. They are also found across the inputs and outputs of DC links.

Low-energy passive filters can use Class 1 ceramic capacitors or silver mica capacitors at high frequencies. Bulk filter smoothing capacitors can be aluminum or tantalum electrolytic capacitors.

Frequency compensation capacitors

Capacitors in conjunction with resistors are used to modify the phase shift and/or amplitude of a transfer function as a function of frequency to provide an adequate phase margin. The phase margin controls the stability of a system (for example, freedom from oscillation) and establishes its dynamic response (for example, overshoot, undershoot and settling time).

Mylar or Teflon capacitors can be used for frequency compensation capacitors.

Motor starting capacitors

A three-phase power source produces a rotating magnetic field, which can be followed (with some slip) by the rotor of an induction motor. However, a rotating magnetic field is not produced inherently in the case of a single-phase source induction motor. Motor starting capacitors come to the rescue.

These motors contain a start winding in addition to the main winding, and an initial rotating magnetic field is developed using the start winding with a series connected start capacitor. The current flowing through the start winding (with the capacitor) produces a 90o phase angle difference (ideally) compared to the current flowing through the main winding. Due to this phase angle difference, a resultant rotating stator magnetic field is produced which will rotate the shaft in the desired direction. A centrifugal switch is attached in series with the start capacitor. When the motor reaches sufficient speed, the centrifugal switch opens to disconnect the capacitor and the start winding. This prevents losses and raises energy efficiency.

Overview of a single-phase induction motor with start capacitor. (Image: Author.)

Overview of a single-phase induction motor with start capacitor. (Image: Author.)

Motor starting capacitors should be non-polarized electrolytic capacitors, which are formed by placing two polarized aluminum electrolytic capacitors in series back-to-back. Protection diodes are often placed in parallel with each of the capacitors to limit the maximum reverse voltage. During the charge and discharge of the capacitors, the diodes do not affect the operation of the capacitors. The capacitors are typically equal in value (say C) so the total capacitance of the series combination is C/2.

How to form a non-polarized electrolytic capacitor. (Image: Author.)

How to form a non-polarized electrolytic capacitor. (Image: Author.)

Motor starting is more demanding in applications where the motor is started frequently, like in gate control applications or air conditioner compressors. The voltage rating of the capacitor should be about 1.5 times the line voltage.

Power factor correction capacitors

A typical AC power system can be modeled using a lumped resistor, a lumped inductor and a capacitor. These elements are in parallel across the AC voltage source. The resistor represents the resistive losses, and true power (work) demands of the system. The inductive component models the inductance associated with the windings of induction motors and any lightly loaded transformers. While the capacitive component can be derived from synchronous AC machines, it can also be a capacitor introduced for the purpose of power factor correction.

Capacitive current is phase shifted 180o from inductive current. Consequently, capacitive current can be used to cancel inductive current. The goal is to make the equivalent AC power load as purely resistive as possible.

Complex power (S) is the phasor sum of the real power and the net reactive power. Apparent power is the magnitude of the complex power (|S|) and has units of volt-amperes (VA). The real component (P) has units of watts, and the reactive component (X) has units of voltage-amperes-reactive (VARs). The power factor is P/|S|= cosθ where θ is the phase angle between the applied voltage and the net current. The power factor should be close to unity (0.85 – 0.97).

Modern polypropylene film power capacitors are state of the art for power factor correction. However, their long-term commercial use is limited to temperatures of less than 85° C. Higher temperature operation requires more expensive glass capacitors.

Power conditioning capacitors

Power conditioning capacitors are connected in parallel with the DC power supply. These capacitors smooth out voltage variations as the load current demands vary. In the case of line-operated DC power supplies, these capacitors smooth out variations in the DC voltage produced by the pulsating DC developed by the rectifiers. These capacitors shunt away power-line hum (for example, 120 Hz) before it gets into the signal circuitry.

The power conditioning capacitors hold up the DC power supply level during brief AC power line interruptions and ensure the minimum instantaneous voltage is large enough to avoid voltage regulator dropout.

Aluminum electrolytic and tantalum electrolytic capacitors are common choices for power conditioning.

Pulsed power capacitors

Pulsed power capacitors are energy discharge capacitors designed to provide high peak discharge current, high energy density, low inductance and low equivalent series resistance. Typical applications include radar, pulsed laser, defibrillators and x-ray equipment.

Pulsed power capacitors are often polymer film and paper capacitors.

Resonant circuit capacitors

Resonant (tuned) circuits usually provide filtering and frequency selection in RF (radio frequency) applications. They are used extensively in RF oscillator circuits and tuned amplifiers.

These applications require capacitors that provide precision and stability. Class 1 (NPO/COG) ceramic capacitors and silver mica capacitors are often used in resonant circuits.

Run capacitors

In single-phase motor applications, capacitors with values above 70 µF are starting capacitors. Run capacitors (typically 3 to 70 µF) are designed for continuous duty and are energized the entire time the motor is running. Start capacitors are used to provide starting torque and establish the direction of rotation. They are switched out by a centrifugal switch as the motor comes up to speed. Run capacitors tend to have smaller capacitance and higher voltage ratings.

A run capacitor is used in single-phase motors to maintain a running torque by using an auxiliary coil. The auxiliary winding helps to maintain running torque when the motor is loaded. These capacitors are considered continuous duty while the motor is powered and will remain in the circuit while the start capacitor drops out.

If a run capacitor is sized incorrectly, the rotating magnetic field will be uneven. This can result in uneven motor rotation speed. This condition worsens as the load increases. The motor losses will increase producing an increased temperature rise and noise.

While non-polarized electrolytic capacitors are used as start capacitors (see above), they are designed for intermittent duty. Run capacitors are used for continuous duty, so low-loss polymer capacitors are often employed.

Single-phase induction motor with run capacitor. (Image: Author.)

Single-phase induction motor with run capacitor. (Image: Author.)

Safety capacitors (EMI/RFI suppression capacitors and AC line filter capacitors)

Safety capacitors are placed across the AC power line to suppress electromagnetic interference (EMI) and high-frequency radio frequency interference (RFI). Should these capacitors fail, they are designed to fail in a safe mode, meaning their failure will not lead to personal injury nor equipment damage.

Safety capacitors are also called EMI/RFI suppression capacitors and AC line filter capacitors. There are two classifications: Class X and Class Y. Class X capacitors are connected across the power line from the hot line to the neutral. Class Y capacitors are connected between the hot line and earth ground and between the neutral ground and earth ground connection.

While connecting the capacitors across the AC power connections permits them to mitigate EMI/RFI noise, it also subjects the capacitors to harsh power line conditions like over voltages and over voltage transients, such as those produced by lightning strikes and power surges. Capacitor failure becomes a significant possibility.

The most common catastrophic capacitor failure mode is for it to become a short circuit. In the case of Class X (hot-to-neutral) capacitors, if it becomes a short circuit, the overcurrent protective device will open. Therefore, capacitor failure will not produce any shock hazards.

Class Y (hot-to-ground and neutral-to-ground) capacitors can also fail due to power line stresses. A short circuit failure could pull the chassis to the hot-line voltage level or corrupt the safety ground connection. These could permit fatal electric shock. Consequently, these capacitors are designed to fail as open circuits. Obviously, this capacitor failure will permit EMI/RFI to creep into the system, but is otherwise a safe failure mode.

Safety capacitors can also be included in line-operated equipment to provide EMI/RFI rejection. They are given a subclass designation of Class X1 and Class Y1 for industrial equipment and Class X2 and Y2 for household equipment.

Ceramic and film capacitors are the fundamental types used for Class X and Class Y capacitors. Ceramic capacitors provide higher capacitor values in a smaller volume, and are usually selected for low-power applications because of their smaller size. Film capacitors exhibit self-healing capability (see above section on EMI/EMC capacitors). Higher capacitance safety capacitor values are only found in film technology.

Class X and Class Y safety capacitors. (Image: Author.)

Class X and Class Y safety capacitors. (Image: Author.)

Snubber capacitors

A simple snubber circuit consists of a capacitor in series with a small-valued resistor. Some also incorporate a switching diode to minimize losses. The purpose of a snubber circuit is to slow the rate of rise in the voltage (dv/dt) across a solid-state switch. Snubber capacitors are used to absorb energy to eliminate the voltage spikes and ringing caused by a switch opening under inductive loads. Snubber capacitors are used across mechanical switches, bipolar junction transistors (BJTs) in power applications, insulated gate bipolar transistors (IGBTs), and thyristor devices like insulated gate commutated thyristors (IGCTs). Snubber capacitors are also used in switch mode power supplies (see above section on buffer capacitors.)

The best choices for snubber capacitors are class 2 ceramic capacitors and metal or plastic film capacitors. Film capacitors are selected because of their low self-inductance, high peak current and low ESR, which are all critical factors in a snubber design. Polypropylene film capacitors are often used in snubber circuits. Self-healing is a recommended characteristic for snubber capacitors.